A giant isopod is any of the almost 20 species of large isopods (crustaceans distantly related to the shrimp and crabs, which are decapods) in the genusBathynomus. They are thought to be abundant in cold, deep waters of the Atlantic,[2]Pacific and Indian Oceans.[1]Bathynomus giganteus, the species upon which the generitype is based, is often considered the largest isopod in the world, though other comparably poorly known species of Bathynomus may reach a similar size (e.g., B. kensleyi).[1]

Giant isopods are of little interest to most commercial fisheries owing to the typical scarcity of catches and because ensnared isopods are usually scavenged beyond marketability before they are recovered. The species are noted for resemblance to the common woodlouse or pill bug, to which they are related. The few specimens caught in the Americas and Japan with baited traps are sometimes seen in public aquaria.

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Giant isopods are a good example of deep-sea gigantism (cf.giant squid). B. giganteus reaches an average length between 19 and 36 centimetres (7.5 and 14.2 in),[3] with a maximum weight and length of approximately 1.7 kilograms (3.7 lb) and 76 centimetres (30 in) respectively.[5] Though most other species of Bathynomus apparently are smaller,[1] they are nevertheless far larger than the "typical" isopods that range in size from 1 to 5 centimetres (0.39 to 1.97 in). Their morphology resembles that of their terrestrial cousin, the woodlouse: their bodies are dorso-ventrally compressed, protected by a rigid, calcareous exoskeleton composed of overlapping segments. Like the woodlouse, they also possess the ability to curl up into a "ball", where only the tough shell is exposed. This provides protection from predators trying to strike at the more vulnerable underside. The first shell segment is fused to the head; the most posterior segments are often fused as well, forming a "caudal shield" over the shortened abdomen (pleon).[3] The large eyes are compound with nearly 4,000 facets, sessile and spaced far apart on the head.[6] There are two pairs of antennae.

The uniramousthoracic legs or pereiopods are arranged in seven pairs, the first of which are modified into maxillipeds to manipulate and bring food to the four sets of jaws. The abdomen has five segments called pleonites each with a pair of biramouspleopods; these are modified into natatory legs and rami, flat respiratory structures acting as gills. The isopods are a pale lilac in colour.

Giant isopods are important scavengers in the deep-sea benthic environment; they are mainly found from the gloomy sublittoral zone at a depth of 170 metres (560 ft) to the pitch darkness of the bathypelagic zone at 2,140 metres (7,020 ft), where pressures are high and temperatures are very low – down to about 4 °C (39 °F).[7] A few species from this genus have been reported from shallower depths, notably B. miyarei between 22 and 280 metres (72 and 919 ft).[8] Over 80 percent of B. giganteus are found at a depth between 365 and 730 metres (1,198 and 2,395 ft).[9] They are thought to prefer a muddy or clay substrate and lead solitary lives.

Although generalist scavengers, these isopods are mostly carnivorous and feed on dead whales, fish, and squid; they may also be active predators of slow-moving prey such as sea cucumbers, sponges, radiolarians, nematodes, and other zoobenthos, and perhaps even live fish. They are known to attack trawl catches. As food is scarce in the deep ocean biome, giant isopods must make do with what fortune brings; they are adapted to long periods of famine and have been known to survive over five years without food in captivity.[10][11] When a significant source of food is encountered, giant isopods gorge themselves to the point of compromising their locomotive ability. One study examining the contents of 1651 giganteus' intestines found that fish were most common there, followed by cephalopods and decapods, particularly carideans and galatheids.[3]

In 1990, the Scavengers of East Australian Seas expedition (SEAS) started to document the scavenging crustaceans along the east coast of Australia by setting traps. The deeper the water, the fewer number of species they found and the larger the species tended to be. The giant isopods found in very deep waters off Australia were compared to those found off Mexico and India. From the fossil record it is thought that Bathynomus existed more than 160 million years ago, before the break-up of the supercontinent Pangaea, so it did not evolve independently in all three locations, but since then it would be expected that Bathynomus would show divergent evolution in the various locations. However, the SEAS study found that the giant isopods in all three locations were almost identical. Andrew Parker in his book In the Blink of an Eye (from where this description of the SEAS expedition is taken) links this lack of evolution to the extremely low light levels of their habitat.[12]

A study of the seasonal abundance of B. giganteus juveniles and adults suggests a peak in reproductive capacity in the spring and winter months. This is observed to be due to a shortage of food during the summer. Mature females develop a brood pouch or marsupium when sexually active, the pouch being formed by overlapping oostegites or brood plates grown from the medial border of the pereopods. The young isopods emerge from the marsupium as miniatures of the adults, known as mancae. This is not a larval stage: the mancae are fully developed, lacking only the last pair of pereopods.